This project is aimed at development of an enzyme immunoassay (EIA) for antibodies to Babesia for use as a clinical diagnostic test. Babesiosis is an emerging tick-borne parasitic disease which may cause severe to fatal illness in immunocompromised or otherwise weakened patients, and may be carried in the blood subclinically in up to 1% of the population in endemic areas. Recent cases of fatal transfusion-transmitted babesiosis have also led to the identification of this pathogen as a significant threat to the blood supply. However, currently no commercial, validated and FDA approved tests are available for B. microti. Babesiosis is currently diagnosed by immunofluorescence staining, microscopy of blood smears, and/or PCR. None of these procedures are easily adaptable to routine clinical laboratory use. We propose to develop a microplate-based ELISA using novel synthetic B. microti antigens licensed to Immunetics that have previously been shown to be diagnostically significant markers. Specificity of the antigens will be assessed in blood donor sera from nonendemic areas and other controls. Assay performance will be evaluated on well-characterized sera from babesiosis patients from both U.S. Northeast and Midwest endemic regions. To identify immunodominant epitopes that may further improve assay sensitivity, we have identified a series of putative Babesia antigenic sequences that will be screened with babesiosis patient serum in an overlapping peptide library format, a strategy that has led to discovery of a number of antigens of diagnostic value. We also propose strategies to identify immunodominant epitopes of other pathogenic Babesia species, B. divergens/MO-1 and B. duncani/WA1. Novel immunodominant epitopes discovered through these strategies will be synthesized as peptide antigens with an appropriate structure for incorporation into current assay formats, which enable combination of multiple distinct peptides in a single-well assay. Once a single-well assay format is developed with sensitivity and specificity characteristics consistent with clinical diagnostic requirements, we will prepare for clinical trials and FDA submission in Phase II.